There is some change in output pressure of a regulator depending on the input pressure change. It is usually a percentage of the input change and depends on the design of the regulator. A typical value could be 1%. So a regulator that is set to give 1100psi when the tank is full at 4500psi will only give 1070psi when the tank has 1500psi left. If the gun was a poppet valve running directly off the tank, it would only represent slightly less than a 3% change in valve pressure and therefore less than 9fps drop in velocity at 300fps. Now, the same 30psi change going into a second regulator such as a mag valve, would represent a 1% change as well. The change in chamber pressure would only be 0.3psi. There would be more inconsistency in the friction of the orings and paintballs than in a 0.3psi change due to air tank pressure in this situation.
So translating that over to a change from a 1100psi tank to a 800psi tank, a 300psi drop in input pressure would represent a 3 psi drop in chamber pressure which is about a 0.6% change in pressure for a level 10 valve system operating at 500psi.. It takes more pressure change at the high end of the velocity range than at the low range due to friction having a greater effect at the higher range. A 0.6% change at 300fps would translate into 1.8fps without friction, but it will be less than that due to frictional forces. Therefore, it would not be a concern because the velocity change when changing tanks would be less than 1.8fps.
Except for the Automag in front, its usually the man behind the equipment that counts.